Many luminescent materials are capable of producing detectable emissions (i.e., output radiation of relatively high spectral energy) in the infrared, visible, and/or ultraviolet portions of the electromagnetic spectrum upon excitation of the materials by appropriate external energy sources. When a luminescent material emits radiation, the emission occurs over a discrete span of time, which may be defined by a measurable decay rate expressed as a decay half-life (i.e., a time period for an emission to reach 50 percent of its peak intensity) or some other quantity. Materials typically described as “fluorophors” (or “fluorescent”) exhibit very short emission decay rates, with half-lives in the micro-, nano- or pico-second range. Conversely, materials typically described as “phosphors” exhibit longer decay rates, with half-lives ranging from several milliseconds to minutes or more (e.g., up to many hours). When the wavelength of a sufficiently-intense emission is in the visible region of the electromagnetic spectrum and its decay half-life is long enough (e.g., greater than the temporal threshold for human perception), the emissions can be observed by the human eye as having a color and brightness (or intensity). Once the excitation has been discontinued, the emissive color decays as a diminishingly observable afterglow, which eventually dies out completely.
A typical luminescent phosphor compound includes at least a host material (e.g., a crystalline composition or crystal lattice), an emitting ion, and in some cases, a “sensitizing” ion (e.g., an ion that can absorb and transfer excitation energy to an emitting ion). The production of radiation by a phosphor compound is accomplished either by absorption of incident radiation (also referred to as “excitation energy”) by an emitting ion and radiation of the energy by the emitting ion, or by absorption of incident radiation by either or both the host material and a sensitizing ion, followed by energy transfer from the host material/sensitizing ion to the emitting ion, and subsequent radiation of the transferred energy by the emitting ion.
The selected components of a phosphor compound may cause the compound to have particular emission properties, including specific wavelengths for its excitation energy, and specific spectral position(s) for its emissions. For a specific phosphor compound that produces observable emissions, the spectral position(s) of the higher spectral energy content (or luminescent output) in its emissions (i.e., its “spectral signature”) may be used to uniquely identify the phosphor compound from other compounds. Primarily, the spectral signature is due to the particular emitting ion(s) included within the phosphor compound. However, spectral perturbations may be present due to the influence of the host material on the various emitting ions, typically through crystal field strength and splitting. This holds true for the temporal behavior of the emissions, as well.
The unique spectral properties of some phosphor compounds make them well suited for use in authenticating or identifying articles of particular value or importance (e.g., banknotes, passports, biological samples, and so on). Accordingly, luminescent phosphor compounds with known spectral signatures have been incorporated into various types of articles to enhance the ability to detect forgeries or counterfeit copies of such articles, or to identify and track the articles. For example, luminescent phosphor compounds have been incorporated into various types of articles in the form of additives, coatings, and printed or otherwise applied features that may be analyzed in the process of authenticating or tracking an article.
An article that includes a luminescent phosphor compound may be authenticated through human observation and/or using specially designed authentication equipment. More particularly, a manufacturer may incorporate a known phosphor compound (e.g., an “authenticating” phosphor compound) into its “authentic” articles. As mentioned previously, phosphor compounds having emission wavelengths in the visible portion of the electromagnetic spectrum with sufficiently long decay half-lives may be observable by the human eye. Such a phosphor compound may be excited using a stationary or portable excitation source that produces excitation energy that is absorbable by the phosphor compound. When the excitation source is removed, the phosphor compound emits light of a particular wavelength for a period of time (e.g., a few seconds). When the emitted light is of an expected color, the human observer may deem the article to be authentic. Conversely, when the phosphor compound emits light of an unexpected color or fails to emit light of a sufficient intensity, the human observer may consider the article to be unauthentic (e.g., a forged or counterfeited article).
Suitably-configured authentication equipment may be used to detect the presence of phosphor compounds having emission wavelengths in the ultraviolet, visible, and infrared portions of the electromagnetic spectrum. When used for authentication, the authentication equipment has knowledge (e.g., stored information and/or a variety of spectral filters) of the wavelengths of absorbable excitation energy and the spectral properties of emissions associated with an authenticating phosphor compound. When provided with a sample article for authentication, the authentication equipment exposes the article to excitation energy having wavelengths that correspond with the known wavelengths of absorption features of the luminescent phosphor compound that lead directly or indirectly to the desired emissions. The authentication equipment senses and characterizes the spectral parameters for any emissions that may be produced by the article. When the spectral signal of detected emissions is within the authenticating parameter range of the detection apparatus that corresponds with the authenticating phosphor compound (referred to as the “detection parameter space”), the article may be considered authentic. Conversely, when the authentication equipment fails to sense signals expected within the detection parameter space, the article may be considered unauthentic.
The above-described techniques are highly effective at detecting and thwarting relatively unsophisticated forgery and counterfeiting activities. However, individuals with the appropriate resources and equipment may be able to employ spectrometry techniques in order to determine the components of some phosphor compounds. The phosphor compounds may then be reproduced and used with unauthentic articles, thus compromising the authentication benefits that may otherwise be provided by a particular phosphor compound. Accordingly, although a number of phosphor compounds have been developed to facilitate article authentication in the above-described manner, it is desirable to develop additional compounds, unique ways of using such compounds with articles, and techniques for authenticating articles, which may render forgery and counterfeiting activities more difficult, and/or which may prove beneficial for identifying and tracking articles of particular interest. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.